Boing Boing recently highlighted Disney's classic 1929 Silly Symphony "Skeleton Dance," which was the cartoon that really drew attention to the studio and to Disney and Ub Iwerks, his chief animator.
Silly Symphonies were among the best cartoons of their time, truly groundbreaking stuff both in terms of the animation style and the density of humor. They changed the pace of gags in cartoons in the 1930s in the same way "The Simpsons" changed the pace of gags in sit-coms in the 1990s.
There are a lot of great Silly Symphonies, which Disney produced for a decade, and a lot of those are on YouTube. Below is a recent favorite of mine, the 1931 short "Egyptian Melodies." It's a very busy and surprising little film that experiments with 3D perspective that looks like a 1980s 3D maze game. A spider wanders down into a pyramid and is emotionally scarred by watching four fully-wrapped mummies dance. The rest of the film is taken up by a great scene played out in the hieroglyphics on the walls. Highly recommended!